The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to an apparatus and method of 3D time-of-flight (TOF) MR angiography.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
MR angiography (MRA) is an imaging technique that is commonly used to image blood vessels and Circle of Willis imaging. TOF-MRA is an MRA imaging technique that relies on the fact that the stationary tissues are saturated and the incoming blood has bright signal from the fresh spin. This is also referred to as in-flow enhancement. One skilled in the art will appreciate that the penetration of blood into the imaging volume depends on the T1 relaxation time of the blood, its velocity, and direction of flow. The effectiveness of MRA is largely predicated upon the degree of contrast achieved between the stationary or static background tissue and the inflowing blood. That is, for the reconstructed image to be generally diagnostically valuable for the identification and detection of pathologies, detectable contrast between the inflowing fluid and the background tissue must be present.
For improved image quality of MRA images, the combination of several saturation pulses is typically employed. The saturation pulses usually include a fat saturation (fatsat) pulse, a magnetization transfer (MT) pulse, and a spatial saturation pulse. The fatsat pulse is used to suppress peripheral fat signal. The MT pulse is used to achieve darker background contrast and the spatial saturation pulse is used to suppress the signal from targeted tissue (arteries or veins). Utilizing all three saturation pulses is effective in improving image quality; however, if all three pulses are played out, the pulse sequence can be prohibitively time-consuming for clinical application. This is illustrated in FIG. 1.
FIG. 1 schematically illustrates a conventional 3D TOF-MRA pulse sequence wherein all three of the fatsat, MT, and spatial saturation pulses are played out. That is, pulse sequence 2 is constructed to have four separate and distinct segments that are repeated every repetition time (TR). At the onset of each TR, a fatsat pulse segment 4 is played out. Immediately thereafter, a spatial saturation pulse segment 6 and an MT pulse segment 8 are played out. Following the MT pulse segment, an imaging segment 9, typically consisting of a frequency encoding (kx) pulse, a phase encoding (ky) pulse, and a slice encoding (kz) pulse, is played out. During each TR, a single k-space line, along the frequency encoding (kx) dimension, is filled up. Also, for both the phase encoding (ky) and slice encoding (kz) dimensions, the k-space data is sequentially acquired from the minimal value (−kmax) to the maximal value (kmax−1). While reasonably effective, the cumulative scan time becomes unquestionably long and, thus, limits the applicability of pulse sequence 2 in clinical applications.
Because the pulse sequence illustrated in FIG. 1 is too temporally long to have clinical applications, in practice, conventional TOF-MRA studies utilize a pulse sequence that recognizes a long out-phase TE to decay the fat signal. As a result, such studies avoid using the fatsat pulse to reduce scan time. However, while effective in reducing scan time, such a solution is susceptible to signal voids caused by turbulent flow dephasing.
It would therefore be desirable to have an apparatus and method capable of TOF-MRA wherein a fatsat pulse, a MT pulse, and a spatial saturation pulse are played out to improve image quality but without the long scan times that have been heretofore required. It would also be desirable to have an imaging technique applicable for TOF-MRA that is less susceptible to signal void artifacts typically caused by flow dephasing.